This invention relates to an arrangement for improving the efficiency of converting DC input power to radio-frequency output signal of a radio transmitter.
The importance of reducing the waste of energy takes on continually increasing importance. To this end, highly efficient solid-state components have been used in radio transmitters, although in some cases the output stages continue to be the somewhat less efficient thermionic vacuum tubes. In amplitude-modulation transmitters, it is known to use a radio-frequency source by an amount dependent upon the energizing voltage applied across the radio-frequency (RF) power amplifier. The magnitude of the energizing voltage is varied at an audio rate in order to achieve audio modulation of the RF signal. The energizing voltage is varied at an audio rate by an audio-frequency power amplifier, and large amounts of audio-frequency modulating power are required in order to modulate a high-power radio-frequency amplifier.
Class A audio amplifiers have a theoretical maximum conversion efficiency of DC power to AC signal of 50%, and class B amplifiers have a theoretical maximum efficiency of 78.5%. Where large amounts of audio power are involved, significant amounts of energy may be wasted as heat dissipation in the audio amplifier. This has been corrected in the past by the use of so-called class D amplifiers, in which the audio signal to be amplified pulsewidth or pulse-duration modulates a subcarrier. The width-modulated pulses are used to drive a high-power switch-mode amplifier to produce high-power pulses at the subcarrier rate, the duty cycle of which varies at the audio information rate. A low-pass filter coupled to the power switching amplifier integrates the duty-cycle modulated signal to remove the carrier and to reproduce the audio signal. Such amplifiers have a theoretical conversion efficiency approaching 100%.
As described in U.S. Pat. No. 4,122,415 issued Oct. 24, 1978 to A. C. Luther, et al., such switching audio amplifiers may introduce a distortion of the modulating signal during those intervals in which the modulation process produces short-duration pulses having a low duty cycle. This distortion results from the finite switching time of the audio power switch transistors. The aforementioned Luther, et al. patent describes an arrangement for correcting the distortion. The correction arrangement includes a threshold device in the form of a diode coupled to the filter and to an offset voltage source. In one of the embodiments shown in the Luther et al. patent, the offset voltage device is a zener diode. During those intervals in which the power switch is open, the reaction voltage resulting from current flow through the inductances of the low-pass filter of the audio amplifier cause a current flow through the threshold diode and through the zener diode which raises the average voltage applied to the filter in such a manner as to compensate for the modulation distortion caused by the switching time of the switched transistors of the switch-mode amplifier. Another embodiment of the offset voltage source includes a capacitor for storing energy in the form of charge resulting from current flow through the threshold diode, together with a resistor for allowing the voltage across the capacitor to be reduced during the intervals between current pulses. In still another embodiment, the offset voltage source includes a battery or an additional power source. The zener diode and capacitor-resistor embodiments of the offset voltage source dissipate power during operation. The power dissipated may be substantial. The battery embodiment of the offset voltage source is undesirable for reliability reasons, but more importantly it also dissipates power, because a resistor parallel with the battery is necessary to prevent destruction of the battery by increasing without limit the energy stored therein during operation. The power supply embodiment mentioned in the specification of the Luther et al. patent as being equivalent to the battery embodiment is subject to the same disadvantages. A more efficient and reliable radio transmitter is desirable.